gruffalo

Hi,

I'm in general happy with my brakes, but when I step really hard on them, I'm not happy with the amount of travel and force it takes to lock the wheels. It seems like there is no more linear response or correspondence between pedal force and brake effect after a certain point.

And the other day (christmast eve actually, thanks to global warming), after a spirited run, I let the car idle, and I pressed on the brake hard (parked), and surely, the pedal kept on sinking at an, not alarming, but not reassuring either, rate. Well, I think I could have gotten it all the way to the floor in 5-10 seconds approx.

Most cars do this to some extent, but will an MC rebuild improve things?

And what does an MC rebuild involve?

MM

littleball_s4

IMO, you have two different problems, not enough resistence to fade and a MC which seals are not 100%. The former is the one that bothers you, the latter, you just found it by chance.

A very old fluid can boil surprisingly fast and, as the brakes fade, you get that non-linear long-travel pedal. Very old pads or rotors may also bit ok, but then fade very fast.

Lousy way, can be cheap and effective: buy a little can and bleed the system. You take out the part that lived in the calipers and also a bit of air.

Proper way, flush the entire system with new fluid. I did it last week (along with new discs and pads front) and the slightly spongy feeling is pretty much the same -I suspect there's air still in there-, but without fade: no more non linear long travel pedal to stop the car at the second hard braking.

MC is obviously not tight, but I think replacing it will not solve the problem with the strange pedal.

gruffalo

I have fresh fluid, and newish disks/pads.

littleball_s4

Ah.

So all you can do is:

Check the system while bleeding it again (in case a caliper is not working or something)

Check pads and disc wear properly and discs match the side.

Go for the MC.

(keep in mind the system is not perfectly linear because of the brake bias valve. Below 18 bar, each bar you increase in your pedal goes all round, above 18 only front).

gruffalo

When bleeding, I noticed that I need to have the reservoir totally full, or else air will be sucked in.

I bleed using pedal, and with the reservoir cap of.

Is this a related issue?

jon928se

Gruffalo

I assume you have no brake fluid leaks anywhere - flexi pipes to the calipers or the calipers themselves - if the fluid level in the reservoir stays constant you don't have leaks.

Try the brake pedal very hard with the engine off (at home with the car stationary). If it slowly goes to the floor you probably have master cylinder seal problems.

But with enough air in the fluid all sorts of strange things can happen.

As your 928 is now 26 years old - you don't say when it might have had brake fluid flushes, I bet the system is full of crap.

I'ld buy a gallon (4.5 litres) of the cheapest DOT3 brake fluid you can buy and flush that through the system. You will also need a pressure bleeder - a pressurised container that you fill with brake fluid that attaches to the master cylinder reservoir cap , then gets pressurised normally by a separate connection to the tyre valve or has it's own pump - you can make your own using a garden bug sprayer and a new reservoir cap - if you use a pressure gauge keep the pressure below about 25psi or 1.7 bar.

The best way to get new fluid in and crap out is to just blow it through - before you connect it all up and pressurise it is useful to completely remove each bleed nipple from the calipers, find a drill bit the same size as the hole down the centre of the nipple and twist it in using your fingers then pull out to pull out the crap with it, repeat until you can see the drill bit when you look through the little holes at the side of the nipple so you know the nipple is clear. then put the nipples back in and presurise the system. It should be obvious where the fluid is not getting through - at about 20 psi the brake fluid should come out of the nipple with enough force to hit the wheel arch (but don't let it sit on the paint) If it doesn't you can remove the nipple completely and see if it just pisses out everywhere . If that doesn't work poke a piece of wire down the hole where the nipple was to dislodge any crap that is in the way.

When all the nipples work really well and you have nice clean clear brake fluid coming through thencheck it out and see if the problems still exist. If they do you have wasted some cheap brake fluid. If there are other problems fix them and then when all is good change the brake fluid for some good stuff - I don't know if you can get DOT 5.1 in Norway that is good, but DO NOT USE DOT 5.

Daniel Dudley

Master cylinders usually deteriorate slowly, and then let go suddenly, as in NO BRAKES. If this is your problem, and it probably is, be warned. You can never be too safe. In a life full of beaters, I have had 3 MCs give out on me. Brakes worked OK, and then in one stop, no brakes. Surprize ! There are two sets of seals in the typical MC, for two circuits. Usually one set lets go, and the other set has to do all the stopping. It quickly fails.

I had total brake failure in a parking lot full of people. Imagine a slalom where the cones are people. I don't mean to sound scary, but I do mean to scare you. Don't be putting off today what you will need tommorrow.

littleball_s4

Yeah, that happened to me also. Once you suck a big amount of air in (a full pedal stroke, let's say) it takes hours to push it out pumping the pedal. At least, I couldn't do it completely in 20' with a proffesional help. An external pump is a great help, I guess.

WallyP

The master cylinder has two flared rubber cups that push/pressurize the fluid. When these cups wear and/or get hardened from age, they no longer flare out and seal against the cylinder as well. This is first noticeable with light pressure on the pedal - firmer pressure will help flare the cups against the cylinder, giving a better seal.

The first indication for most drivers is a sinking brake pedal while sitting at a traffic light...

The only cure is to replace/rebuild the master cylinder.

Some useful tests for the brakes:

With the engine not running, firmly press and fully release the brake pedal ten or twelve times. Fully release the pedal, wait a few seconds, then firmly press and hold the brake pedal. If the pedal is high and firm, your hydraulic system is probably good. If there is any sponginess, you have air in the lines, a soft hose, or a master cylinder problem, in that order of probability.

After doing that test, press and hold the brake pedal while cranking the engine. The pedal should fall and get softer - doing so indicates that the power booster is working. It doesn't prove that the booster is working correctly, just that it is working.

In an area with no traffic at all, with the engine running, press and release the brake pedal, wait a few seconds, then press and hold the pedal, getting a feel for the feel, and especially the travel. Drive the car, and then let it coast to a full stop without touching the brake pedal. Press and hold the brake pedal - if there is any increase in pedal travel as compared to the non-moving check, you probably have a warped rotor or loose wheel bearings that are pushing the pads away from the rotor.

gruffalo

Wally P: thanks for the advice! All parts on the brakes (on the entire car almost) are 3 years old. A PO rebuilt the whole car, but maybe not the MC. Disks, calipers, hoses all new. Fluid approx 6 months old. I doubt that there is any air in the system, I ran at least 1/2 jar of fluid thru every nipple after the last bubble was seen.

I'll run thru your test scheme, but my money is on the MC for now.

MM

PS I have new pressure switches, and there is no funny business going on with them, no warning light, and brake lights working, so I must have equal pressure in both circuits.